1. Field of the Invention
The present invention relates to a fuel vapor purge system, and more particularly, an apparatus and method of detecting a leak in the fuel vapor purge system.
2. Related Art
According to a conventional fuel vapor purge system, fuel vapor generated from a fuel tank is stored in an active carbon canister. The fuel vapor stored in the active carbon canister is purged to an intake path of an engine and combusted by the engine when a predetermined condition is satisfied. A purge valve is provided between the active carbon canister and the intake path to thereby control an amount of the fuel vapor to be purged. The path is provided with a canister valve.
It is requested for a fuel vapor purge system to check presence or absence of a leak. For example, by hermetically closing a portion of the fuel vapor purge system and monitoring pressure in the hermetically closed path, the leak of the hermetically closed path can be detected. For example, by closing the purge valve and the canister valve, a path including a fuel tank, the canister and a plurality pipes can hermetically be closed. For example, the pressure in the hermetically closed path can be monitored by a sensor for detecting pressure in the fuel tank. Such a leak check can be carried out in operating the engine or when the engine is stopped. U.S. Pat. No. 5,263,462 discloses a leak check which is carried out when the engine is being stopped.
Various new problems are posed in the fuel vapor purge system which can hermetically be closed.
For example, under a state in which the system is hermetically closed, rapid deformation of the fuel tank brings about rapid change in pressure in the fuel tank. Therefore, there is a concern of causing an error in the leak check. Particularly, a fuel tank made of resin is more liable to deform than a fuel tank made of a metal plate. The deformation of the tank may be brought about by a pressure difference needed for the leak check. Therefore, it is also conceivable that the deformation of the tank is brought about at each leak check.
For example, according to the fuel vapor purge system which can hermetically be closed, unpreferable pressure may be maintained. For example, excessive positive pressure or excessive negative pressure is maintained in the fuel tank. Such an excessive pressure exposes a component of the fuel vapor purge system to a severe pressure difference over a long period of time to thereby bring about deformation or deterioration in function. For example, an excessive pressure difference is operated to a fuel tank over a long period of time. In other aspect, negative pressure hampers the leak of the fuel vapor from the fuel vapor purge system to the atmosphere. Therefore, it is preferable to maintain inside of the fuel vapor purge system under negative pressure.